Gregor to Axel: which sense of "plain" - non-formatted, or monospace formatted? This is decisive to me.

I assume that tilde+any means: any, except in a nowiki-inline/pre-block, correct? Gregor Hagedorn

Yes. The only markup recognized in nowiki/pre is the end markup, which can be escaped with other rules (leading space for pre, more than three closing braces at the end of a span in nowiki) --YvesPiguet.